President Trump on Thursday said there was no collusion between his campaign and Russia, and denied ever asking FBI Director James B. Comey to back off on his agency’s investigation into former national security adviser Michael Flynn.

Trump spoke in the wake of Deputy Attorney General Rod J. Rosenstein’s decision to appoint a special counsel to investigate any coordination between Trump associates and Russian officials.

“I respect the move, but the entire thing has been a witch hunt, and there is no collusion between — certainly myself and my campaign, but I can always speak for myself and the Russians. Zero,” Trump said at a joint news conference Thursday afternoon with President Juan Manuel Santos of Colombia. “Believe me, there’s no collusion.”

Asked whether he urged Comey to drop the Flynn investigation — as the fired FBI chief said he did in notes written after a meeting with the president — Trump said, “No, no,” before ordering the media to move on to the “next question.”

Also Thursday afternoon, Rosenstein went to Capitol Hill and indicated to the full Senate that the White House’s initial account of Comey’s firing was not accurate because he said he knew that Comey would be fired before he wrote a controversial memo that the White House initially used as its justification for the dismissal.

(Peter Stevenson/The Washington Post)

Rosenstein did not reveal any more significant details about the Comey firing, his decision to appoint a special counsel or the ongoing Russia investigation during the unusual 90-minute closed briefing with most of the 100 senators, according to interviews with several senators afterward.

Rosenstein did, however, emphasize to the senators the independent authority that the new special counsel — former FBI director and federal prosecutor Robert S. Mueller III — has in the Russian investigation.

“If one thing is clear from the meeting we just had, it is that Mr. Mueller has broad and wide-ranging authority to follow the facts wherever they go,” said Senate Minority Leader Charles E. Schumer (D-N.Y.). “That gives me confidence and should give the American people some confidence.”

Although the meeting was held in a secure room in the Capitol Visitors Center where classified information can be discussed, nothing Rosenstein shared with the senators was “remotely classified,” according to one senator who asked for anonymity to speak frankly about the meeting.

“He could have shared what he told us in a public hearing,” the senator said.

Because of Mueller’s wide scope in the Russian probe, Rosenstein referred several of the senators’ questions to the new special counsel, frustrating many of the senators who wanted to learn more.

Rosenstein “was very careful about not going into any details surrounding the removal because he wants to give Robert Mueller the opportunity to make an independent decision” about how to proceed, Sen. Claire McCaskill (D-Mo.) said as she emerged from the briefing.

Rosenstein received strong support from the Senate a month ago when he was confirmed by a vote of 94 to 6 to be the Justice Department’s second-highest-ranking official. But his reputation has come under fierce attack in the past week over the memo he wrote about Comey.

Since Comey’s firing May 9, the calls for Rosenstein to appoint a special counsel intensified, especially from Democratic lawmakers who said he could no longer be impartial in the Russia investigation. Rosenstein had been put in charge of the probe as soon as he was confirmed because Attorney General Jeff Sessions recused himself after The Washington Post reported on contacts he had with the Russian ambassador that he had not disclosed when asked about them during his Senate confirmation hearing.

Rosenstein did not notify White House counsel Donald McGahn of his special counsel decision until 5:30 p.m., the same time Justice Department officials were briefing reporters and 30 minutes before the news became public. Senior congressional aides said that some lawmakers were also given a heads-up in advance of the White House.

In the Senate meeting, each senator was given the opportunity to ask one question. Several Democrats asked multiple questions, and some Republicans took a pass.

Rosenstein told the senators that, in fact, Trump had decided to fire Comey the day before he wrote his memo.

“To me, it was significant that he stated that he knew that the decision to fire Comey had been made the day before he drafted the memo,” said Sen. Angus King (I-Maine).

Why Rosenstein felt compelled to write the memo remains unknown. Sen. Richard J. Durbin (D-Ill.) said that Rosenstein told the senators that he was not pressured into writing it.

“He learned the president’s decision to fire him and then he wrote his memo with his rationale,” Durbin said.

According to a person close to the White House, Rosenstein was upset about the narrative that emerged from the White House on the evening of May 9. White House officials cast Rosenstein as the prime person behind the decision to fire Comey, even though Trump had already decided to terminate the director. Rosenstein threatened to resign from the Justice Department because of the explanation that White House officials were giving reporters about the firing, said a person close to the White House, who spoke on the condition of anonymity.

By May 10, White House officials had backed off blaming Rosenstein for the firing, and the next day, Trump contradicted his own officials and told NBC News that the decision to fire Comey was his alone and that he was thinking of “this Russia thing with Trump” when he made it.

During Thursday’s news conference, Trump contradicted both his own account and that of Rosenstein. “Director Comey was very unpopular with most people,” Trump said. “I also got a very, very strong recommendation, as you know, from the deputy attorney general, Rod J. Rosenstein.”

The president also expressed surprise that he had not received bipartisan support for his decision to fire Comey. He called the suggestion he had done anything potentially worthy of criminal charges “totally ridiculous.”

Trump had earlier in the morning lashed out on Twitter at the news of the special prosecutor, calling the move a politically motivated “witch hunt” by his Democratic rivals. The president’s anger contrasted with a more measured written statement released by the White House on Wednesday evening, when Trump declared that a thorough investigation would find “no collusion between my campaign and any foreign entity.”

Several Republican senators asked Rosenstein whether the Senate Intelligence Committee could continue its own investigation into Russian interference in the 2016 election now that Mueller has been appointed special counsel on the same matter. Rosenstein was “unequivocal” that the panel can and should continue its investigation, according to people familiar with his remarks.

Sen. John Cornyn (R-Tex.), who just removed himself from consideration to be the next FBI director, said that senators are taking the federal investigation “enormously seriously.”

“Clearly Russia was very much involved in trying to undermine public confidence in our elections,” Cornyn said. “I don’t think it’s in anybody’s interested to delay or impede or impair this investigation in any way. We need to be focused on what our role is. We are not the FBI or the Department of Justice. We are conducting oversight.”

Rosenstein did find time for some levity.

“Somebody asked, how do you pronounce your last name? He said it’s Rosen-STINE,” Sen. Bill Cassidy (R-La.) said, adding that Rosenstein referred senators to a recent NPR segment that taught listeners how to properly pronounce his name.

This report has been changed to correct an inaccurate rendition of President Trump’s remarks. He said, “I respect the move, but the entire thing has been a witch hunt, and there is no collusion between — certainly myself and my campaign, but I can always speak for myself and the Russians. Zero.” He did not say,”I can only speak for myself and the Russians.

Kelsey Snell, David Nakamura and Karoun Demirjian contributed to this report.